This invention relates to a fuel injection control device for a two stroke internal combustion engine and more particularly to an improved method of determining the air flow to such an engine for controlling the amount of fuel injected.
It is well known that the fuel requirements for an internal combustion engine are dependent upon the speed at which the engine operates and also the load on the engine. Although speed is a relatively easy characteristic to measure, load is not so easy to determine. In the crudest form of load measurement, it has been proposed to merely measure the position of the throttle valve of the engine in order to determine its load. However, the throttle valve position can be subject to fluxuation and its position per se is not always an indication of the instantaneous load on the engine. In addition, some forms of engines such as diesel engines do not have throttle valves and hence there is no throttle valve position to sense.
Air flow is a more accurate way of measuring the load on the engine inasmuch as the air flow to the engine is a relatively accurate indication of its load. In addition, it is desirable to maintain a certain fuel/air ratio and in order to maintain that, the amount of air consumed must be determined.
Various devices have been proposed for positioning in the induction system for measuring the air flow to the engine. These devices are such as rotating vanes or electrically heated wires which provide some indication of air flow. However, any device which is positioned in the induction system tends to restrict the air flow to the engine and can adversely affect its performance.
It has been found that the air flow to a two cycle, crankcase compression engine can be very accurately measured by measuring the difference in pressure in the crankcase at the time of scavenge port opening and at scavenge port closing. This pressure difference is a very accurate indication of the amount of air consumed by the cylinder per cycle. This can easily be measured by placing a crankcase pressure sensor in the crankcase and timing the taking of the pressure signals in relation to the timing of the scavenge port opening and closing.
The actual timing at which the pressure is sensed is important to having an accurate determination of air flow. However, the types of systems previously proposed have had some defects because of certain inherent lag in the system. For example, the construction of the sensor per se introduces a time lag in that the pressure output trace from the sensor does not completely accurately follow the actual crankcase pressure due to the inherent operation of the sensor. The effect of this will be described later by reference to FIG. 3.
Another defect inherent with the prior art type of constructions is that the pressure sensor is normally electrically operated and is driven by the charging coil of the ignition system of the engine. This is particularly true with outboard motor applications wherein a separate source of electric power may not be provided. The variation in output timing is thus dependent not only on the inherent construction of the sensor, which normally provides a fixed offset, but also at the speed which the engine is running, which provides a variable offset. This variable offset will be described later by reference to FIG. 4.
It is, therefore, a principal object to this invention to provide an improved device for measuring engine air flow by measuring crankcase pressure.
It is a further object to this invention to provide an improved fuel control for a two cycle, crankcase compression, internal combustion engine.
It is another object to this invention to provide a pressure sensor for sensing crankcase pressure wherein the pressure signal is sensed at the appropriate time to provide the required data.
In addition to the aforenoted problems, there is also the problem of deterioration of the signal from the pressure sensor during the life of the apparatus. This problem is particularly acute in conjunction with outboard motor applications wherein the air inducted into the engine may contain a large amount of water vapor and which water vapor may be salt water when operating in marine environments. This water vapor can impinge upon the detecting surface of the pressure sensor and if corrosion occurs, then the pressure signal will further deteriorate. It is difficult to provide a continuous correction for such deterioration, which can vary depending upon the particular application.
It is, therefore, a still further object to this invention to provide an improved pressure sensor arrangement for a two cycle, crankcase compression, internal combustion engine wherein the detecting surface of the pressure sensor will be protected from corrosion, even in marine environments.
It is a still further object to this invention to provide an improved pressure sensor for a two cycle, crankcase compression, internal combustion engine.